Fire safety systems include, among other things, detection devices and notification devices. Detection devices include smoke, heat or gas detectors that identify a potentially unsafe condition in a building or other facility. Detection devices can also include manually operated pull stations. Notification devices, often referred to as notification appliances, include horns, strobes, and other devices that provide an audible and/or visible notification of an unsafe condition, such as a “fire alarm”. Many fire control systems include loudspeaker devices that provide an audible signal generated at a source. The loudspeaker devices may be used to announce trouble, testing or other notifications to the inhabitants of a building.
In commercial, industrial, and multiple-unit residential buildings, fire safety systems can include one or more fire control panels that serve as distributed control elements. Each fire control panel may be connected to a plurality of distributed detection devices and/or a plurality of distributed notification appliances. In addition, other field panels or circuits operate to provide audio signals to at least one loudspeaker. As with other notification appliances, loudspeakers can distributed throughout an entire facility.
In a typical loudspeaker configuration, low voltage audio signals are provided to an amplifier within a field panel or field circuit. The amplifier transmits the audio signal to one or more loudspeakers via suitable wires. To allow for extended distribution without excessive loss, relatively high voltage audio signals are employed. For example, it is known to use 25 volt, 70 volt, and 100 volt audio signals in fire safety audio circuits. Relatively high voltage signals are less prone to I2R losses, and provide less sound degradation.
In large facilities, multiple distributed amplifiers provide audio signals to loudspeakers. Accordingly, the low voltage “source” audio signal must be conveyed to different amplifiers throughout the facility. Because the low voltage audio signal is subject to losses in long propagation paths, it is difficult to provide audio signal announcements throughout large systems with loudspeakers distributed over substantial areas in a building or campus.
One solution provided to this problem is to tap audio signals from the high voltage audio circuit that feeds the speakers. In this configuration, the high voltage audio circuit provides high voltage audio signals to loudspeakers, and then to a downstream circuit that receives the signal. The downstream circuit is a device that, for example, re-amplifies the signal to produce another high voltage audio signal for additional speakers. In this configuration, it is necessary to isolate the high voltage audio signals of the “source” circuit from the downstream circuit. Such isolation is required because conductors of significant length can be susceptible to unwanted common mode signals and noise. Such isolation is typically carried out using a transformer or capacitor coupling
A drawback of the above-described solution is that it presents difficulties in providing continuity information and fault detection. In particular, it is known to use a DC voltage signal as a supervisory signal in various portions of a fire safety system to test for continuity. The supervisory signal is transmitted when an audio signal (or other alarm signal) is not being transmitted. Control circuitry detects aspects of the supervisory DC signal to determine if a short circuit or open circuit has occurred. In the above-described arrangement, the isolating transformer cannot pass the supervisory DC signal through to downstream circuits.
While downstream circuits can generate and analyze their own supervisory signals, there is not currently a way for information regarding upstream supervisory signals to be propagated.